Field
Disclosed herein are diene elastomer compositions reinforced with an inorganic filler, such as silica, intended for the manufacture of tires or tire semi-finished products, in particular for the treads of these tires.
More particularly, disclosed are coupling agents intended to provide the bonding, in such compositions, between these diene elastomers and these reinforcing inorganic fillers.
Description of Related Art
Major efforts have been made by tire designers, so as to reduce the consumption of fuel and the pollution emitted by motor vehicles, in order to obtain tires simultaneously exhibiting a very low rolling resistance, an improved grip both on a dry surface and on a wet or snowy surface, and a good wear resistance.
This has been made possible in particular by virtue of the development of novel elastomer compositions reinforced with specific inorganic fillers, described as “reinforcing”, which exhibit a high dispersibility, which are capable of competing with conventional carbon black from the reinforcing viewpoint and which additionally afford these compositions a reduced hysteresis synonymous with a lower rolling resistance for the tires comprising them. Such rubber compositions, comprising reinforcing inorganic fillers, for example of the silica or alumina type, have been described, for example, in Patents or Patent Applications EP 501 227 or U.S. Pat. No. 5,227,425, EP 735 088 or U.S. Pat. No. 5,852,099, EP 810 258 or U.S. Pat. No. 5,900,449, EP 881 252, WO99/02590, WO99/06480, WO00/05300, WO00/05301 and WO02/10269.
The processability of the rubber compositions comprising such fillers nevertheless remains more difficult than for the rubber compositions conventionally comprising carbon black as filler. In particular, it is necessary to use a coupling agent, also referred to as bonding agent, the role of which is to provide the connection between the surface of the particles of inorganic filler and the elastomer while facilitating the dispersion of this inorganic filler within the elastomeric matrix.
It should be remembered here that (inorganic filler/elastomer) “coupling” agent has to be understood, in a known way, as meaning an agent capable of establishing a satisfactory connection, of chemical and/or physical nature, between the inorganic filler and the diene elastomer.
Such a coupling agent, which is at least bifunctional, has as a simplified general formula “Y—W—X”, in which:                Y represents a functional group (“Y” functional group) which is capable of being physically and/or chemically bonded to the inorganic filler, it being possible for such a bond to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, the surface silanols, when silica is concerned);        X represents a functional group (“X” functional group) capable of being physically and/or chemically bonded to the diene elastomer, for example via a sulphur atom;        W represents a divalent group which makes it possible to connect “Y” and “X”.        
The coupling agents should in particular not be confused with simple covering agents for the inorganic filler which, in a known way, may comprise the Y functional group, active with regard to the inorganic filler, but are in any case devoid of the X functional group, active with regard to the diene elastomer.
Coupling agents, in particular (silica/diene elastomer) coupling agents, have been described in a large number of patent documents, the most well known being silane bifunctional sulphides, in particular alkoxysilanes, regarded today as the products contributing, for vulcanisates comprising silica as filler, the best compromise in terms of scorch safety, of ease of processability and of reinforcing power. Mention should very particularly be made, among these silane sulphides, of bis(3-triethoxysilylpropyl)tetrasulphide (abbreviated to TESPT), the reference coupling agent in tires with a low rolling resistance described as “Energy-saving Green Tires”.